In current wood golf clubs and in metal wood golf clubs, excessive side spin is generated to the ball on all off-center hits on the clubface. The resulting horizontal curve of the golf ball is usually called gear-effect and is an inherent characteristic for all golf clubs in which the weight distribution allows excessive rotation of the clubhead about its vertical axis through its center of gravity for all off-center impact with the golf ball.
Improper weight distribution and incorrect center of gravity location in any wood or metal wood type golf clubhead assembly requires the use of bulge radius to help correct for these deficiencies in golf clubhead designs.
Bulge radius is used as a side loft angle on the face of the wood and metal wood clubheads to start the ball further off the target line to correct for the excessive part of the gear-effect curve, and the ball will land on the fairway but with some loss in distance, due to the excessive side spin and curve.
The U.S.G.A. Rules of Golf reads "The face shall not have any degree of concavity." However, the rules allow a convex surface thus allowing bulge and roll radii and also a flat face.
A review of the major golf clubhead catalog for 1991 shows a total of twenty-four different designs of metal wood type golf clubheads and only three wood golf clubheads for a total of twenty-seven different designs and all twenty-seven list a bulge radius.
The need to use bulge radius to make these golf clubheads playable indicates that the current wood type metal golf clubs do not have, or do not claim, optimum weight distribution and a matched center of gravity location for the clubhead necessary to control gear-effect to the extent of being able to use a flat clubface.
The center of gravity distance measured from the the moment arm about which the clubface rotates for all off-center impact with the golf ball.
The high impact force and friction between the ball and the rotating clubface causes a horizontal spin component on the ball resulting in a horizontal curve in the flight of the golf ball called gear-effect.
When the moment arm or center of gravity distance is too great and not matched to the weight distribution of the clubhead, excessive horizontal spin is generated to the ball and the gear-effect is not adequately controlled and therefore bulge radius must be used on the clubface to make the golf club assembly playable.
The amount of bulge required to keep the ball on the fairway is determined by driving range tests. That is the procedure used to design present wood and metal wood type golf clubs.
All the designs reviewed to date use a bulge radius and also a roll radius.